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Natural Geochemical Enrichments of Elements

GLY 4241 - Lecture 3

Fall, 2014

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Important Low-Abundance Elements• Elements used in steel alloys

Vanadium, chromium, nickel, niobium

• Elements used in rechargeable batteries Nickel, cadmium, lithium

• Jewelery Gold, silver, platinum, palladium

• Nuclear fuels Uranium, thorium

• Miscellaneous Copper, used in wiring, plumbing, alloys Mercury, used in electrical switches, pesticides, fluorescent bulbs,

etc.

Need for Concentration

• Most of these elements could not be mined, processed, and formulated into useful products at reasonable costs if they occurred everywhere at their average abundances in the crust

• For example, millions of tons of gold exist in seawater, but the cost of obtaining pure gold from seawater is many times the value of the gold

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Enrichment

• We need to answer two questions: 1. For any given element, how much

enrichment above natural abundance values is needed to produce a mineable ore?

2. What geochemical processes are responsible for producing these natural elemental enrichments?

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Definition of Ore

• The naturally occurring material from which a mineral or minerals of economic value can be extracted at a reasonable profit (from the Glossary of Geology, 3rd edition)

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Factors Influencing Cost of Metals

• Exploration

• Mining Rights Acquisition

• Cost of mining, Includes cost of compliance with existing

environmental regulations

• Ore separation and processing

• Transportation of ore to consumer

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Frank Wigglesworth Clarke, 1847-1931

• Early career involved teaching, including a year at Howard University, and 9 years at the Univ. of Cincinnati

• Later, Chief Chemist, USGS, 1883-1924

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Definitions

• Clarke = the average abundance of an element in the crust of the earth

• Clarke of concentration = the concentration of an element in a rock compared with its average concentration in the earth's crust, or of an element within a particular mineral

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Clarke of Concentration

• Clarke of copper is about 55 ppm, or 0.006%

• In the mineral chalcocite, Cu2S, the Cu concentration is 79.8%

• Thus, the clarke of concentration within this mineral is 79.8/0.006, or 13,300

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Clarke ValuesMetal Clarke Minimum metal %

for profitab leextraction

Clarke ofConcentration

Al 8.13 30 4

Fe 5.00 20 4

Mn 0.10 35 350

Cr 0.01 30 3000

Cu 0.006 0.25 40

Ni 0.0075 1.5 200

Zn 0.007 4 600

Sn 0.0002 1 5000

Pb 0.0013 4 3000

U 0.0002 0.1 500

Ag 0.00001 0.05 5000

Au 0.0000005 0.0005 1000

Early Earth

• The early earth was probably bombarded by solid planetesimals, primarily chondritic meteorites

• Chondritic meteorites may be left over from the protoplanet stage of the solar system

• Chondritic meteorites are composed of three different phases, or combinations of these phases

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Chondrite Phases

• Nickel-iron metal

• Iron sulfide

• Silicates, largely olivine or pyroxene

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Exchange Reactions

• M + Fe silicate ↔ M silicate + Fe

• M + Fe sulfide ↔ M sulfide + Fe

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Goldschmidt Element Affinities

• Siderophile: Elements concentrated in the metallic phase, along with metallic iron

• Chalcophile: Elements concentrated in the sulfide phase

• Lithophile: Elements concentrated in the silicate phase

• Atmophile: Elements concentrated in the atmosphere

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Meteorite Phases

• Iron-nickel metal

• Troilite (sulfide)

• Silicate

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Geochemical Classification of Elements

Siderophile Chalcophile Lithophile Atmophile

Fe* Co* Ni* (Cu) Ag Li Na K Rb Cs (H) (C) N (O)

Ru Rh Pd Zn Cd Hg Be Mg Ca Sr Ba (Cl) (Br) (I)

Os Ir Pt Ga In Tl B Al Sc Y REE He Ne Ar

Au Re+ Mo+ (Ge) (Sn) Pb Si Ti Zr Hf Th Kr Xe

Ge* Sn* W++ (As) (Sb) Bi P V Nb Ta

C++ Cu* Ga* S Se Te O Cr U

(P) As+ Sb+ (Fe) Mo (Os) H F Cl Br I

(Ru) (Rh) (Pd) (Fe) Mn (Zn) (Ga)* Elements are chalcophile and lithophile in the earth's crust.+ Elements are chalcophile in the earth's crust++ Elements are lithophile in the earth's crust() Elements show affinity for more than one group. Secondary group(s) are shown in parentheses.After Mason and Moore (1982); Brownlow (1979)

Trace Elements

• Working definition: element whose concentration is less than 0.1%

• May form their own minerals, but typically are too scarce to do so

• Typically, trace elements will follow a major element into another mineral, where they replace part of the major element

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Siderophile Characteristics

• Elements whose valence electrons are not readily available for combination with other elements

• Positive charge on the nucleus, at least under certain conditions, exerts a strong attraction on the outer electrons, preventing combination

• These elements usually occur in the native state

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Chalcophile Characteristics

• Elements whose valence electrons may be shared, but are not electropositive enough to donate electrons or electronegative enough to accept electrons

• Thus, the bonds formed are predominantly covalent

• Since sulfur is much less electronegative than oxygen, sulfur is prone to form covalent bonds with these elements

• Generally the chalcophile elements have their valence electrons outside a shell of 18 electrons

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Lithophile Characteristics

• Elements that are strongly electropositive or electronegative and thus typically donate or accept electrons, forming ionic bonds

• Most silicate minerals have oxygen ions that can form ionic bonds to metal cations

• Generally the lithophile elements have their valence electrons outside a shell of eight electrons

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Atmophile Characteristics

• Elements that do not readily combine with other elements, or which form diatomic molecules held together in the solid or liquid states only by very weak Van der Waal forces

• All of the inert gases, with completed shells or subshells, fall into this category

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Oxygen

• Secondary atmophile element would not occur in the atmosphere of the earth if the earth were at chemical equilibrium

• Oxygen is maintained in the atmosphere only by the continual photosynthesis within the biosphere

• Indeed, the presence of oxygen in an atmosphere is often regarded as an indicator of life on the planet

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Atomic Volume vs. Atomic Number

• Vertical scale should be atomic volume